With the present explosive increase in wireless communications, available unreserved radio communications frequencies are rapidly decreasing. For example, in the United States, the Federal Communications Commission (FCC) has restricted a range of radio communications channels to government or military use. Other radio communications channels are licensed for commercial use, however a great many of these are already licensed by existing commercial radio stations, cellular communications providers, television broadcasters, and so forth. A similar situation exists in many developed countries of the world.
While a particular radio communications channel may be licensed for use throughout a particular territorial jurisdiction, such as the United States, it is likely that such channel is not in fact used all of the time in every geographical part of the jurisdiction. For example, while a commercial radio station may broadcast on a licensed frequency in one or more major metropolitan areas, its radio signals may not reach more remote areas, or areas obstructed by geographical features such as mountains and large buildings. Similarly, the commercial radio station may choose not to broadcast during certain times of the day, for example in the period between midnight and four o'clock AM, when the broadcast may not reach enough listeners to justify the costs of programming.
As a result, there are radio communications frequencies that are in fact available for opportunistic use without substantially interfering with communications transmitted by the license holder.